Production line systems for applying coatings to such articles as cookware vessels are well-known. In general, such systems include a conveying means for bringing the article to be coated to the coating station, means for coating the article at a coating station or in a coating zone, and conveying means for removing the just-coated article at the conclusion of the treatment in the coating station or zone. All such systems fall short of an ideal system in one or more respects however.
At this writing, standard industry practice includes the use of operator initiated means, almost always manual labor, to properly position the pan to be coated for the following spraying operation. If the interior of a pan is to be coated by spraying, for example, the fingers of the line attendants must touch and handle the pan to orient it into the proper position. Such touching and physical manipulating can give rise to imperfections in the final product, such as nicks and dents. Indeed, even the placement of fingerprints on the smooth surface which is to be coated can eventually appear as an imperfection in the final coated surface, the end result of which may be rejection of the pan as not commercially saleable in view of the high demands for quality which characterize present day consumers. Even in those instances where no physical degradation of the product due to manipulation preparatory to coating is present, the cost of the manipulation steps adds a substantial increment of cost to the final cost.
In addition to the drawbacks of cost and possible quality defects attendant to the preparation for spraying, many current systems generate additional costs attributable to inefficient spraying. For example, many current systems use a mask system to preclude the sprayed coating from reaching surfaces which are not to be coated. The use of a masking step in the process adds costs, complexity and, possibly, time, depending of course on the extent to which an upstream pan can be masked while a downstream pan is being sprayed so that the process times overlap. Usually, masking is done on the pan to be sprayed after it reaches the spraying station and just prior to spraying, thereby requiring that the spray gun or guns be inactivated while masking is carried out.
Further, many current systems present the pan to the spraying station in a shell up position. This has the disadvantage that the interior shell surface which is to be sprayed will collect tramp materials, such as flecks of solids which float in the area of the immediate environment of the pan, or even water. The presence of foreign substances such as water or flecks of dirt or dust on the surface to be sprayed may produce undesirable surface imperfections which will cause the end product to fail to meet quality standards.
Standard industry installations frequently include a continually moving chain which carries the pan to be sprayed past a stationary spray nozzle. In such a system the spray nozzle is programmed to commence spraying at a time prior to the arrival of the leading edge of the continuously moving pan, and to terminate spraying at a time after the pan passes the spray nozzle. The commencement of spraying before the pan arrives and continuation of spraying for a period after the pan passes the spray nozzle is necessary in order to ensure that all areas on the pan, and particularly the leading edge and the trailing edge of the pan relative to the spray nozzle, are coated. In addition there will be considerable overspray in those installations in which the spray pattern of the spray nozzle or nozzles is arranged to project a spray which impinges on the maximum diameter of the pan at all times. Thus as a consequence of the early on--late off nozzle actuation period, at least, large quantities of make-up air are required as contrasted to the quantity which would be needed if the pan was not axially moving during spraying so that the initial droplets of spray were assured of hitting a receiving surface and the spray could be terminated as soon as a bare surface was no longer presented to the spray. Make-up air is invariably drawn from a location outside the plant in which the spraying installation is located. In summer, the make-up air is almost always at a temperature which is suitable for use in the spraying operation and hence no conditioning of the outside make-up air is required. However, in the winter in northern climes, the outside air must be heated to bring it to a temperature suitable for use in the spraying operation. The heating of large quantities of cold, outside air to properly condition the air for use in the spraying operation is expensive. Hence, it is desirable that a spray system require the minimum possible quantity of make-up air. The use of a lesser quantity of make-up air will, of course, result in a reduced quantity of exhaust air when it is appreciated that the order of magnitude of make-up air required in a current standard installation is on the order of 5,000-10,000 cubic feet per booth per hour.
And finally, overspray of the coating material is a concern in many current installations from the standpoint of efficiency of utilization of the coating material and the cost of maintenance of the system, including filters. All systems use an enclosed chamber in which spraying is done, but nearly always overspray is controlled by means of a single source of suction, usually a fan. Fan sizes can be very large of course but, as a practical matter, capital and operating costs place a practical limit on the spray withdrawal capacity, and often that limit is not high enough to ensure that virtually all of the overspray will enter the withdrawal system. In addition, in most current systems, a very small percentage, at best, of the overspray is collected and re-used due to the diffused character of the overspray particles by the time they reach the filtering mechanism in the withdrawal system flow path.
It has also been observed that conveyor maintenance is a substantial cost item in many current coating systems. Specifically, in many systems the pan is coated while in contact with the conveyor. If a pan is coated on the conveyor a conveyor cleaning device must be added to the system to ensure that pans entering the system will not be fouled by the residue left from prior spraying operations. In other systems in which an attempt is made to spray the pan at a location remote from the conveyor, complicated and costly lifting and handling systems are employed, which systems again raise the possibility of creating imperfections in the final products as well as increasing the cost of production.
For these reasons, among others, no current spraying system can be considered to be the ultimate, or near ultimate system, and the possibilities of cost reduction and quality improvement remain as challenges to the users of such systems.